Karzai announced the widely expected delay during Secretary of State Condoleezza Rice's first visit to Afghanistan, and insisted that security was improving, despite a bomb attack that killed five civilians in the southern city of Kandahar.
The parliamentary vote had been slated for May but the United Nations and the Afghan electoral commission have been grappling with problems including a lack of census data and how to register thousands of returning refugees.
"The preparations are going on and now they told us, the commission chairman, that the elections will be held in September," Karzai said at a news conference with Rice at his Kabul palace. "The Afghan people are waiting very eagerly to send their members to parliament."
Observers have long said the election, initially scheduled along with a presidential ballot last year, would be put off because of the huge task of organizing it. The election is also supposed to produce district and provincial assemblies.
Afghanistan adopted a new constitution early in 2004 and successfully held the presidential vote in October, despite worries over violence. The parliament vote is seen as the next step in the country's democratic re-birth after a quarter-century of conflict.
Rice said the United States would support Afghanistan as it prepares for the vote and called its re-emergence from years of war an inspiration to the world.
The country's booming illegal narcotics industry was a "serious problem," Rice said, though one which both she and Karzai said was being tackled with a U.S.-backed crackdown on opium poppy farmers and smugglers through millions in aid to promote alternative crops.
"We will stand by the Afghan people as they go through the next stage in their democratic development, the parliamentary elections that will take place this fall. We look forward to continuing to help in the reconstruction of Afghanistan," she said.
She said the United States made a mistake by losing its focus on Afghanistan following the Soviet withdrawal in 1989. The country plunged into years of civil war after the withdrawal, allowing the hardline Taliban to take power and turn the country into a safe haven for Osama bin Laden and his al Qaeda terror network.
"We have a long-term commitment to this country," said Rice, who was making her first visit to Afghanistan.
"We learned the hard way what it meant to not have a long-term commitment," she said. "After the Soviet Union left I think it is well understood that we did not remain committed, and I said to the president earlier that in many ways Sept. 11 was a joint tragedy of the Afghan and American people out of that period."
Rice called Afghanistan a "great ally," and said the nation's turnaround is one of the world's greatest stories. "This country was once a source of terrorism. It is now a steadfast fighter against terrorism," Rice said.
Rice pledged Washington's continued support for Afghan democracy and reconstruction, but made no public mention of whether Washington would support permanent American bases in the country.
Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said during a visit to the country on Wednesday that no decision had been reached on whether to seek permanent bases on Afghan soil.
Earlier Thursday, Rice shook hands with U.S. troops at their headquarters in Kabul and thanked them for helping to liberate Afghanistan. Some 17,000 U.S. forces continue to battle Taliban and al Qaeda fighters in the tribal belt between Afghanistan and Pakistan, where Osama bin Laden may be sheltering.
Commanders insist the militants are waning, but also warn of an upturn in violence as the harsh Afghan winter fades.
Police blamed Taliban rebels for Thursday's attack in downtown Kandahar, which killed at least five people and wounded 32. The blast hit a taxi carrying women and children, a roadside restaurant and other bystanders.
American and Romanian forces cordoned off the area around the blast in a busy commercial district crowded with shops and restaurants. The shoes and turbans of the wounded were scattered on the bloodstained street, along with the wreckage of the taxi, a motorized rickshaw and two motorbikes.
A purported Taliban spokesman denied responsibility, pointing the finger instead at militia commanders disgruntled at a U.N. disarmament drive.
Afghanistan is Rice's third stop on a six-nation Asian trip. She arrived in Kabul from Pakistan. She visited India earlier, and will travel to Japan, South Korea and China.